Investigate why cygdrives don't show up with listdir

[lazka]

Came up in chat:

• Why is there no C when doing ls /. In cygwin those are visible, under ls /cygdrive at least.

[zhangxiaoxing]

This could be annoying at times because in the (somewhat) popular nnn terminal file manager, the user are expected to navigate through whatever they can see. It's quite common that I randomly moved to the msys2 root and got stuck there, because the entire file-system is invisible from there.

[lazka]

Someone in chat mentioned

I seem to remember it was an intentional performance-related change specifically when cygdrives are mounted on the root.

[zhangxiaoxing]

Some further digging revealed this to be a 10+ year old confounding observation that rooted in cygwin/cygdrive. I'm trying to go back to the more conservative mount point of /cygdrive/X but so far editing /etc/fstab and restart msys2 UCRT64 shell doesn't seem to work.

[dscho]

The reason is that the opendir() function (that is called implicitly by ls /) calls into fhander_disk_file::opendir(), not into fhandler_cygdrive::opendir().

Looking at the code, it strikes me as unplanned to have the cygdrive-mount directory be actually read (see fhandler_cygdrive::opendir(): It totally ignores disk contents). Compare that to fhandler_dev::opendir() which very clearly prefers to look at on-disk files, but falls back to faking a directory. The difference gets even more pronounced in the readdir() method, where the fhandler_dev variant first reads the on-disk files, then iterates over the emulated devices and finally the disks/partitions. In contrast, there is nothing at all about real on-disk files in fhandler_cygdrive::readdir().

There's another tell-tale indicating that Cygwin never considered / to be the cygdrive: there are a couple of explicit cygdrive_len > 1 guards, e.g. the one introduced in 3409763.

So I don't think that this is a bug, but it looks like a deliberate design decision on Cygwin's part, and therefore I would expect it to be non-trivial to make the cygdrives show up as part of ls /.

Of course, it would be always possible to imitate fhandler_dev::readdir() in fhandler_disk_file::readdir(), guarded inside an if (mount_table->cygdrive_len == 1 && pc.get_posix() == "/") { ... } block, to specifically append virtual directory entries after the regular on-disk entries have been enumerated. This would happen right here. To hold the necessary state, we would need to extend the __DIR_mounts class (an instance of which is available via d_mounts(dir)).